Selecting Windows
for Energy Efficiency

New window technologies have resulted in greater energy benefits and more practical options for homes. This selection guide will help homeowners and designers take advantage of the expanded window market.

An understanding of some basic energy concepts is essential to choosing appropriate windows and skylights. Three major types of energy flow occur through windows, as shown in Figure 1: (1) non-solar heat losses and gains in the form of conduction, convection, and radiation; (2) solar heat gains in the form of radiation; and (3) airflow, both intentional (ventilation) and unintentional (infiltration). (See the "Window Energy Glossary" for explanations of these terms.)

The non-solar heat flow through a window is a result of the temperature difference between the indoors and outdoors. Windows lose heat to the outside during the heating season and gain heat from the outside during the cooling season, adding to the energy needs in a home. The effects of non-solar heat flow are generally greater on heating needs than on cooling needs because indoor-outdoor temperature differences are greater during the heating season than during the cooling season in most regions of the United States.

A U-factor, or U-value, is a measure of the rate of non-solar heat flow through a window or skylight. (The commonly used term R-value is a measure of the resistance to heat flow and is the inverse of the U-factor.) U-factors allow consumers to compare the insulating properties of different windows and skylights.

The insulating value of a single-pane window is due mainly to the thin films of still air on the interior and exterior glazing surfaces. The glazing itself doesn't offer much resistance to heat flow. Additional panes markedly reduce the U-factor by creating still air spaces, which increase insulating value.

In addition to conventional double-pane windows, many manufacturers offer windows that incorporate relatively new technologies aimed at decreasing U-factors. These technologies include low-emittance (low-e) coatings and gas fills.

A low-e coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited on a glazing surface. The coating may be applied to one or more of the glazing surfaces facing an air space in a multiple-pane window, or to a thin plastic film inserted between panes. The coating limits radiative heat flow between panes by reflecting heat back into the home during cold weather and back to the outdoors during warm weather. This effect increases the insulating value of the window. Most window manufacturers now offer windows and skylights with low-e coatings.

The spaces between windowpanes can be filled with gases that insulate better than air. Argon, krypton, sulfur hexafluoride, and carbon dioxide are among the gases used for this purpose. Gas fills add only a few dollars to the prices of most windows and skylights. They are most effective when used in conjunction with low-e coatings. For these reasons, some manufacturers have made gas fills standard in their low-e windows and skylights.

The insulating value of an entire window can be very different from that of the glazing alone. The whole-window U-factor includes the effects of the glazing, the frame, and, if present, the spacer. (The spacer is the component in a window frame that separates glazing panes. It often reduces the insulating value at the glazing edges.)

Since a single-pane window with a metal frame has about the same overall U-factor as a single glass pane alone, frame and glazing edge effects were not of great concern before multiple-pane, low-e, and gas-filled windows and skylights were widely used. With the recent expansion in glazing options offered by manufacturers, frame and spacer properties have a more pronounced influence on the U-factors of windows and skylights. As a result, frame and spacer options have also multiplied.

Window frames can be made of aluminum, steel, wood, vinyl, fiberglass, or composites of these materials. Wood and vinyl frames are far better insulators than metal. Insulated fiberglass can perform slightly better than either wood or vinyl. Some aluminum frames are designed with internal thermal breaks, non-metal components that reduce heat flow through the frame. These thermally broken aluminum frames can resist heat flow considerably better than aluminum frames without thermal breaks. Composite frames have insulating values intermediate between those of the materials comprising them. Frame geometry also strongly influences energy performance.

Spacers can be made of aluminum, steel, fiberglass, foam, or combinations of these materials. Spacer energy performance is as much a function of geometry as of composition. For example, some well-designed metal spacers insulate as well as foam. Table 1 shows representative U-factors for window glazing, frame, and spacer combinations.

Due to their greater projected surface areas, domed and other shaped skylights have significantly higher U-factors than vertical windows of similar materials and opening sizes.

Air can hold varying amounts of water vapor or moisture. The warmer air is, the more moisture it can hold. The amount of moisture in the air, expressed as a percentage of the maximum amount the air could hold at a given temperature, is called its "relative humidity." For health and comfort, indoor air should contain some moisture. The relative humidity should generally be between 30% and 40% at normal room temperature.

The relative humidity of air can be increased by adding more moisture or by reducing the temperature. When the relative humidity reaches 100%, the air can hold no more moisture, and water begins to condense from it. The temperature at which this condensation occurs is called the "dew point temperature" of the air. When moist air comes in contact with a cold surface in a home, it may be cooled to its dew point temperature, resulting in condensation on the surface.

Windows don't cause condensation; they are simply the first and most obvious place it occurs. This is because windows generally have lower thermal resistances than insulated walls, ceilings, and floors. As a result, their inside temperatures are usually lower than those of other surfaces in a home during cold weather. If the air in a home is humid enough, water will condense from it when it is cooled at a window surface.

Left unchecked, condensation can damage window frames, sills, and interior shades. Water can deteriorate the surrounding paint, wallpaper, plasterboard, and furnishings. In severe cases, it can seep into adjoining walls, causing damage to the insulation and framing.

The indoor air coming in contact with energy-efficient windows is less likely to be cooled to its dew point temperature because the inside surface temperatures remain higher during cold weather than do those of windows with single glazing, traditional metal spacers, and metal frames.

Figure 2 illustrates conditions under which condensation will form on the center of the glass of three glazing types with widely varied U-factors. The graph shows clearly that the risk of condensation at the center of the glass is reduced as the insulating value of the glass increases. Even at an outdoor air temperature of -30[ring]F, the indoor air relative humidity must be nearly 50% before condensation will form on the triple glazing with two low-e coatings. On the other hand, at an outdoor temperature of 10[ring]F, condensation will form on the single glazing at an indoor relative humidity of only 18%.

Condensation is even more likely to occur at window spacers and frames, which are usually less insulating than the corresponding glazings. With so many insulating glazing types available, efforts to prevent condensation have shifted toward the development of better insulating spacers and frames.

Figure 2. Outdoor air temperature and indoor air relative humidity at which condensation will occur on the center of the glass for single glazing, double glazing, and triple glazing with two low-e (E = 0.15) coatings. On or above each curve, the conditions are right for condensation. Below each curve, condensation will not occur on that glazing type as long as the glazing is exposed to room air circulation. (Results are based on winter conditions: 70deg. F indoor air temperature, 15 mph outdoor air velocity, and no incident solar radiation.)

When shopping for windows and skylights, pay close attention to whether the U-factor listed by the manufacturer applies to the glazing only or to the entire unit. If it is for the glazing only, the overall U-factor is probably considerably higher because of the frame and spacer effects. These effects increase with decreasing total window area. Compare different window types or makes by their total U-factors. This information and the steps in "The Difference a U-Factor Can Make," (p. 17) can be used to estimate the relative heating energy usage associated with a particular window type.

Avoid aluminum-frame windows without thermal breaks if possible. Even in milder climates, these windows tend to have low inside surface temperatures during the heating season, giving rise to condensation problems. Aluminum-frame windows with properly designed thermal breaks can be used in moderate climates. Wood, vinyl, and fiberglass are the best frame materials for insulating value.

Single-pane windows are impractical in heating-dominated climates. In these regions, multiple-pane, low-e, and gas-filled window configurations are advisable. (Be aware that, depending on design, construction, and filling method, gas-filled windows may leak over a period of time, somewhat reducing their insulating value.) Remember that lower window and skylight U-factors mean less energy consumption, lower utility bills, and greater comfort in the living space.

Solar transmission through windows and skylights can provide free heating during the heating season, but it can cause a home to overheat during the cooling season. Solar-induced cooling needs are generally greater than heating benefits in most regions of the United States. In fact, solar transmission through windows and skylights may account for 30% or more of the cooling requirements in a residence in some climates.

Because the position of the sun in the sky changes throughout the day and from season to season, window orientation has a strong bearing on solar heat gain. Figure 3 shows the solar heat gain through ¹/₈-in clear single glass for various window orientations on very clear days in the heating and cooling seasons at 40deg. latitude. South-facing windows allow the greatest and potentially most beneficial solar heat gain during the heating season, while admitting relatively little of the solar heat that contributes to cooling requirements during the cooling season. The reverse is true for skylights and east- and west-facing windows. North exposures transmit only minimal solar heat at any time.

A solar heat gain coefficient is a measure of the rate of solar heat flow through a window or skylight. (A shading coefficient is the previous standard indicator of a window's shading ability and is equal to the solar heat gain coefficient multiplied by 1.15.) Solar heat gain coefficients allow consumers to compare the shading properties of different windows and skylights.

Additional glazing panes provide more barriers to solar radiation, thus reducing the solar heat gain coefficient of a window. Tinted glazings, such as bronze and green, provide lower solar heat gain coefficients than does clear glass. Low-e coatings can be engineered to reduce window solar heat gain coefficients by rejecting more of the incident solar radiation. Spectrally selective glazings, including some low-e coated glazings with low solar heat gain coefficients and new light blue and light blue-green tinted glazings, block out much of the sun's heat while maintaining higher visible transmittances and more neutral colors than more heavily tinted glazings.

Table 2 shows representative solar heat gain coefficients and visible transmittances for glazings with typical wood or vinyl frames and aluminum spacers. (Aluminum-frame windows of comparable size and glazing type generally have slightly higher solar heat gain coefficients because of their thinner frames and greater glazing areas.) Multiple glazing panes, tints, and low-e coatings clearly reduce solar heat transmission.

Figure 3.Solar heat gain through 1/8-inch clear single glass for window orientations at 40deg.N latitude (for example, Columbus, Ohio, and Boulder, Colorado).

Ultraviolet light is the main component of solar radiation that can fade and damage drapes, carpets, furniture, and paintings when transmitted through windows and skylights. Efforts to produce window glazings that transmit less ultraviolet light have met with limited success. In general, more glazing panes and low-e coatings reduce ultraviolet transmission through windows and skylights.

Window solar heat gain coefficients should be selected according to orientation. If south exposures are to admit beneficial solar heat during the heating season, their solar heat gain coefficients should be high. These high solar heat gain coefficients will not usually result in overheating problems during the cooling season because of the lower solar radiation levels on south-facing windows, especially those with overhangs, at that time.

Skylights and east- and west-oriented windows may warrant lower solar heat gain coefficients since they transmit the most solar heat during cooling periods. There isn't much point in spending more money to obtain lower solar heat gain coefficients for north-facing windows.

Windows with spectrally selective or low-e coated glazings with low solar heat gain coefficients are often effective in hot, sunny climates. Darker glazing tints also provide lower solar heat gain coefficients, but they may yield somewhat decreased visibility.

If exterior or interior shading devices, such as awnings, louvered screens, sunscreens, venetian blinds, roller shades, or drapes, will be used on windows, lower window solar heat gain coefficients may not be necessary, depending on individual circumstances. Many shading devices can be adjusted to admit more or less solar heat according to the time of day and the season, but windows with lower solar heat gain coefficients require less maintenance.

Exterior shading devices are more effective than interior devices in reducing solar heat gain because they block radiation before it passes through a window. Light-colored shades are preferable to dark ones because they reflect more, and absorb less, radiation. Horizontally oriented adjustable shading devices are appropriate for south-facing windows, while vertically oriented adjustable devices are more effective for shading windows on east and west orientations.

Low-e windows and skylights are the best options for decreasing the transmission of ultraviolet radiation.

Windows, skylights, and glazed doors now bear energy ratings or labels, similar to those placed on household appliances. Before these labels were developed, different energy rating techniques were employed by different window manufacturers. Now homeowners can compare products directly, regardless of glazing, frame, and spacer type. The window energy label lists the U-factor, solar heat gain coefficient, visible light transmittance, and air leakage rating. The rating system was developed and implemented by the National Fenestration Rating Council. (NFRC is a nonprofit coalition of manufacturers, builders, state and federal energy officials, private and government laboratories, utilities, consumers, and others.) The ratings are determined using a variety of advanced computer tools developed in the United States and Canada, combined with actual product performance testing.

Windows, skylights, and glazed doors now bear energy ratings or labels, similar to those placed on household appliances. Before these labels were developed, different energy rating techniques were employed by different window manufacturers. Now homeowners can compare products directly, regardless of glazing, frame, and spacer type. The window energy label lists the U-factor, solar heat gain coefficient, visible light transmittance, and air leakage rating.

The rating system was developed and implemented by the National Fenestration Rating Council. (NFRC is a nonprofit coalition of manufacturers, builders, state and federal energy officials, private and government laboratories, utilities, consumers, and others.) The ratings are determined using a variety of advanced computer tools developed in the United States and Canada, combined with actual product performance testing.

Airflow through and around windows occurs by design as ventilation and inadvertently as infiltration. The use of windows for natural ventilation is as old as architecture itself. Opening windows, particularly on opposite sides of a living space, can cool a home for free. The sash type of a window influences the ventilation airflow rate through the window relative to its size. Some common sash types and their effective open areas for ventilation purposes are shown in Table 3. Casement windows are especially effective for ventilation because they tend to direct the greatest airflow into the living space when fully open.

Infiltration is the leakage of air into a building from the exterior through joints and cracks around window and skylight frames, sash, and glazings. This leakage can account for 5% to 30% of the energy usage in a home. The airtightness of a window depends on the sash type as well as the overall quality of the window construction and installation. Because of the way they seal against the framing, windows with compressing seals are generally more airtight than purely sliding seals.

An air leakage rating is a measure of the rate of infiltration around a window or skylight in the presence of a strong wind. Air leakage ratings allow consumers to compare the airtightness of different windows and skylights as manufactured.

Operable, rather than fixed, windows should be installed in household areas with high moisture production, such as bathrooms, kitchens, and laundry rooms, and in other areas where natural ventilation is desired.

Select windows with air leakage ratings of 0.2 cubic feet per minute per square foot of window area (cfm/ft²) or less. Check the seals between window components for airtightness. To minimize infiltration around installed windows, caulk and weatherstrip cracks and joints.

Jeffrey L. Warner is a senior research associate with the Energy Analysis Program at Lawrence Berkeley Laboratory.

This article was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy of the U.S. Department of Energy and by the Environmental Protection Agency as part of a series on energy-efficient remodeling.